1. Field of the Invention
The invention relates to a process for preparing large-particle, aqueous dispersions, which permits targeted adjustment of particle size. In the present specification, large-particle dispersions denote dispersions in which the particles have a volume-average diameter (dv) in the range of 130 to 1,000 nm, preferably 150 to 800 nm. Dispersions of this type are still flowable after concentration to a solids content of &gt;55% by mass. They are, therefore, considered to be of low viscosity.
2. Discussion of the Background
A number of processes for preparing large-particle, aqueous dispersions have been developed in the prior art, but these all differ from the present process and suffer from a number of shortcomings and disadvantages.
In British Patent 991,394 (Polymer Corp. I), there is described a process for the preparation of large-particle, aqueous plastic dispersions, in which a nonionic, water-soluble polymer auxiliary (agglomerating agent) is added to an aqueous plastic dispersion after polymerization. By distilling off some of the water, plastic dispersions of low viscosity and high solids content are obtained. Preferably the polymer auxiliaries are substantially non-branched and have a molecular structure containing hydrophobic and hydrophilic radicals. Less preferred polymer auxiliaries have an AB block structure with a hydrophobic radical A and a non-branched hydrophilic radical B. These auxiliaries are the monolaurate, monostearate and monooleate derivatives of a polyethylene glycol (polyethylene oxide) having a molecular weight of 6,000. This process can be modified such that the polymer auxiliary is added to the reaction mixture from which the plastic dispersion is obtained. As such, the polymer auxiliary can specifically be added either before or after initiation of the polymerization. However, these auxiliaries are used in an amount of 0.1 parts by weight, based on the weight of total monomers plus sodium sulfate.
In German Offenlegungsschrift (German Published Specification) 2,222,176 (Polymer Corp. II), there is described a process for the preparation of large-particle, aqueous plastic dispersions in which a nonionic, water-soluble polymer auxiliary is added to the reaction mixture, from which the plastic dispersion is obtained, before initiation of polymerization. The polymer auxiliary is substantially non-branched and has a molecular structure containing both hydrophobic and hydrophilic radicals. However, the polymer auxiliaries do not have an AB block structure with a hydrophobic radical A and a hydrophilic radical B. Additionally, the large-particle dispersions of this process are prone to coagulation.
In European Offenlegungsschrift (European Published Specification) 330,865, a process for the preparation of large-particle, aqueous plastic dispersions is described. It permits targeted adjustment of the particle size of the dispersions independently of other target parameters, such as the molecular weight of the polymers, i.e. it permits the adjustment of the volume-average diameter (dv), and the particle size distribution. In particular, targeted adjustment of a narrow, monomodal distribution is possible. This process employs a preferred polymer auxiliary which is an oxidized polyethylene oxide, i.e. a mixture of a non-branched and a branched polyethylene oxide. However, neither of the two constituents of the mixture contain any hydrophobic radicals. Another preferred polymer auxiliary of this process is the reaction product obtained from the trimerization product of isophorone diisocyanate and polyethylene oxide monoethyl ether. This is a mixture in which the main component has a molecular structure containing a branched hydrophobic radical and three hydrophilic radicals. The main component is thus branched and does not have a AB block structure with a hydrophobic radical A and a hydrophilic radical B.
The polymer auxiliaries of European Offenlegungsschrift (European Published Specification) 330,865, which represent the closest prior art, are non-uniform and in the form of a mixture, the composition of which varies, depending on the conditions for the preparation of the polymer auxiliary. Under certain circumstances, the structure of the components of the mixture also varies, depending on the preparation conditions. Accordingly, the agglomerating effect of the polymer auxiliaries varies. They, therefore, do not always meet the stringent requirements demanded in practice regarding the reproducibility of their agglomerating effect. Moreover, they are characterized only inadequately by conventional analytical methods.